During the past ten years, it has become increasingly clear that the inhalation of airborne combustion-related ultra fine particles (UFPs) presents a significant health-hazard to humans, owing to the fact that these particles tend to deposit on and eventually encapsulate in the lung tissue. Such UFPs comprise both solid particles and liquid-like particles. A significant part of the combustion-related solid particles is composed of soot particles that comprise or largely consist of unburned elemental carbon. A smaller part of the combustion-related solid particles is composed of inorganic ashes. Ultra fine combustion-related liquid-like particles are typically composed of more-or-less volatile hydrocarbon/H2SO4/H2O material together with small amounts of inorganic species. Combustion-related UFPs measure approximately between 5 and 2500 nm in diameter, in particular between 5 nm and 500 nm in diameter (most particles measuring less than 200-300 nm in diameter), and normally comprise or are at least partially covered with carcinogenic polycyclic aromatic hydrocarbons (PAHs) and other volatile organic compounds (VOCs). These UFPs are emitted into air from the exhaust of combustion sources such as automobile motors and are formed as the result of an incomplete combustion process. In particular diesel motors are notorious for emitting large amounts of soot particles and other UFPs into air.
Apart from the neighborhood of industrial combustion sources and other stationary combustion sources, the concentration of combustion-related UFPs, hereafter simply referred to as UFPs, in the western world is generally highest on or near locations where motorized traffic is present. Very high local concentrations may be encountered particularly in tunnels, traffic intersections and/or in traffic queues under conditions of limited ventilation and/or windspeed. However also in (rooms of) buildings, recreational cabins, huts, homes, vessels, aircraft, spacecraft, and individual compartments/rooms inside said vehicle cabins, recreational cabins, huts, homes, buildings, vessels, aircraft, and spacecraft, highly health-hazardous concentrations of UFPs may be encountered.
Especially automobile drivers and passengers become readily exposed to elevated concentrations of UFPs and other exhaust pollutants because the vehicle's air handling system (which may e.g. be either a heating, ventilating, air conditioning (HVAC) system or a basic heating/ventilation system) continuously draws outside air, that is polluted by the exhaust gases and particles emitted from the exhausts of other vehicles, into the vehicle cabin. It is therefore desirable to be able to at least partly clean the outside air of various airborne pollutants by means of an air cleaning unit before allowing its entrance into the cabin, and to automatically control the settings of the air handling system in response to conditions pertaining to the outside air, notably the humidity, the temperature and the pollution level, in order to minimize the exposure of the vehicle's inhabitants to air pollutants, while retaining comfortable temperature and humidity levels.
As described in U.S. Pat. No. 5,775,415, the operation mode of the vehicle's air handling system can be controlled by an electrical control unit that actuates and controls the rotation of a switching damper element, positioned between the cabin air inlet and the outside air inlet associated with the air handling system. The switching damper element is rotated such as to fully close the cabin air inlet and to fully open the outside air inlet in the input mode operation, while fully opening the cabin air inlet and fully closing the outside air inlet in the re-circulation mode operation. In the mixed mode operation, the switching damper element can assume a series of intermediate positions that partly open both the cabin air inlet and the outside air inlet such that a controlled amount of re-circulating cabin air and a controlled amount of outside air are simultaneously allowed to enter the air handling system.